1. Field of the Disclosure
The present invention is related to locating casing joints downhole and, in particular, to detecting earth-induced magnetic fields associated with the casing joints.
2. Background of the Art
Forming a casing in a wellbore includes fastening multiple casing tubulars end to end at a downhole location. Once the casing is set in the wellbore, various operations downhole require knowing the location of a casing joint, i.e., a connection between two adjacent casing tubulars. For example, determining the location of the casing joint may be useful to accurately place whipstocks in a cased well so that drilling through the casing joint can be avoided.
One method of locating a casing joint is used on casings with residual magnetic fields therein. The residual magnetic fields are generally the result of magnetic testing of the casing tubulars at an uphole location prior to disposing the casing tubulars in the wellbore. However, casing tubulars that have been de-magnetized or are made of soft magnetic materials do not retain a magnetic field. The material of these non-magnetic tubulars may however be such that a surrounding magnetic field, such as the earth's magnetic field, induces a magnetic field in the casing tubular. However, the character of the magnetic field at the casing joint for a magnetic field induced in the casing tubulars is different from the character of the magnetic field at the casing joint that has a residual magnetic field therein. Therefore, previous magnetic techniques for identifying casing joints are not applicable for casing tubulars having an earth-induced magnetic field.